I have been playing mp4 files ok. Then I decided that flv should run faster so I ran all my mp4 files through convertilla to make them into flv files.
The flv files are in the same folder as the mp4 files but when I changed the tag line form {mp4)tai{/mp4} to read {flv}tai{/flv} I get the error "Error loading player no playable source found.

www.guildfordu3a.org.uk/
The row of .... at the foot of the home page is a secret link to my video test page which has tai.flv set up

You should generally avoid flv files.
They are not supported on the vast majority of handheld devices and some desktop ones as well.

Try with an flv/f4v file that was not generated from convertilla to see if you can reproduce the issue.

Right If I should avoid flv files which is the best format to use please?

@Mike Piper

I suggest to use MP4 videos format to ensure maximum device compatibility.